Vinyltriphenylphosphonium Bromide1

[5044-52-0]  · C20H18BrP  · Vinyltriphenylphosphonium Bromide  · (MW 369.25)

(phosphonioethylation; two-carbon chain-linking and alkenation; ring formation by Michael-Wittig, Michael-nucleophilic displacement, and Diels-Alder sequences1)

Physical Data: mp 189-190 °C. The proton,2 carbon, and phosphorus3 NMR and the mass spectrum have been reported.4

Solubility: sol t-butanol.

Form Supplied in: white powder.

Preparative Methods: preparation of vinyltriphenylphosphonium bromide (1) has been accomplished by treatment of ethylenebis(triphenylphosphonium) dibromide with borate ion (32%),5 (2-chloroethyl)triphenylphosphonium bromide with Triethylamine (72%),6 (2-bromoethyl)triphenylphosphonium bromide with Silver(I) Oxide (85%),7 and (2-phenoxyethyl)triphenylphosphonium bromide with boiling ethyl acetate (92%).2

Purification: may be recrystallized from t-butanol-ether6 to give an analytically pure sample having a mp of 189-190°.2

Handling, Storage, and Precautions: hygroscopic; irritant; produces sneezing in many individuals. Use in a fume hood.

General Discussion.

Phosphonioethylation has been accomplished by employing a variety of nucleophiles (eq 1). Low concentrations (<=1.5 M) of aqueous Sodium Hydroxide give only (b-hydroxyethyl)triphenylphosphonium bromide (2) (RX = HO), whereas greater concentrations (&egt;2.0 M) of hydroxide give (2) (RX = HO) plus products attributable to an initial attack of hydroxide on the P atom.8 A number of other nucleophiles have been employed to give high yields in the phosphonioethylation reactions; these include alcohols,2,9 primary and secondary amines,10,11 nitrosobenzene,12 thiols,2,9,13 and methylene groups activated by electrophilic moieties.14-16

The Michael-type addition of a nucleophile to (1) followed immediately by a Wittig alkenation reaction has sometimes been called the Schweizer reaction (eq 2).17 The nucleophiles (X) include various carbon, nitrogen, and oxygen species, with one example of a trimethyltin nucleophile.18,19 Poor yields are reported for reactions of alkyllithiums (13-16%),20 Phenyllithium (41%),20 and diethyl ethylsodiomalonate (14%)21 with ketones. The reaction of butyl-, vinyl-, and heptynyllithium gives none of the hoped-for product (3) with benzaldehyde.7 However, cuprates give relatively good yields of (3). The (Z)-isomer predominates;22 the addition of 5 equiv of Hexamethylphosphoric Triamide to the reaction enhances formation of the (Z)-isomer.7

Secondary amines,21 pyrrole,23 sulfonamides,21 and phthalimides,17 as their salts, react with (1) and a variety of aldehydes to give allylamines. The (Z)-isomer is formed predominantly unless lithium salts are added.17 The replacement of (1) by vinyltri-n-butylphosphonium bromide gives predominantly the (E)-isomer.16 Primary amines have been successfully used only as the dilithio salts.11 Sodium Ethoxide and thiophenoxide (see Thiophenol) react with (1) and benzaldehyde to yield (3).21

The Schweizer reaction has been used mainly to prepare carbocyclic and heterocyclic species (eq 3).24

The types of ring that have been formed by this Schweizer annulation reaction are illustrated in eqs 4-12 using specific examples. Each equation is preceded by CnXm to illustrate the actual elemental components of the ring formed. The types of ring are: cyclopentene (eq 4);25-27 cyclohexene (eq 5);25,28-30 pyrrole or dihydropyrrole (eq 6);31-40 1,2-dihydroquinoline (eq 7);41 2,3-dihydropyridazine (eq 8);40 dihydrofuran (eq 9);14,42 3,4-chromene (eq 10);43 2,5-dihydrothiophenes (eq 11);44-48 and 3,6-dihydro-2H-thiopyrans (eq 12).49

A novel extension of the Schweizer annulation reaction is a [2 + 2 + 2] annulation reaction,50 involving two consecutive Michael-type additions followed by ring closure.51-53 The reaction has been used successfully employing 1 mol of (1) and another Michael acceptor (eq 13) or 2 mol of (1) (eq 14).53

Ring formation employing (1) has also been accomplished with reaction sequences involving Michael-type additions followed by a nucleophilic internal substitution reaction (eq 15).30,54-58

[3 + 2] Cycloadditions have yielded pyrroles (eq 16),59 2-pyrazolin-3-yltriphenylphosphonium bromides (eq 17),60 pyrazoles (eq 18),61 and the adduct from a nitrone, which decomposes spontaneously (eq 19).62 Diels-Alder, or [4 + 2], cycloadditions have also been explored (eq 20).63-65

Related Reagents.

Allyltriphenylphosphonium Bromide.


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Edward E. Schweizer

University of Delaware, Newark, DE, USA



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